Thermodynamic study of methane multilayers adsorbed on graphite

Abstract

We have made thermodynamic measurements of the properties of methane adsorbed on graphite in the ranges, roughly, 1—6 molecular layers and 64—105 K. Interpreting the results in the context of current models, we tentatively conclude that layer by layer, critical points occur at approximately constant temperature around 78 K. Extrapolation of this observation would imply a roughening transition in bulk solid methane at about that temperature. We also observe a line of melting transitions that extends from the bulk triple point, $T_t=90.7\mathrm{}$ K, into the multilayer region at nearly constant temperature. The transition in films of less than ∼5 layers does not appear to be first order, but it is associated with a change in entropy. The change in entropy gradually diminishes, vanishing entirely at about 2 layers. It is not clear how the transition closes, but a number of possibilities is discussed.